Device for finding the flow rate of a flowing medium

ABSTRACT

A device for determining the throughput of a flowing medium is described, wherein a heater resistor heats at least one upstream and downstream located, temperature-dependent resistor, wherein the resistors, which are cooled at different rates by the flowing medium, are wired to form a bridge, whose diagonal voltage is a function of the temperature difference of the resistors. In the end the mass of the flowing medium is determined from this processed voltage (UM). The control of the temperature of the resistor is performed with the aid of its own heater control circuit which regulates the current through the heater resistor in such a way that the temperature of the heater resistor is a predeterminable function of the temperature of the flowing medium. A temperature compensation, with the aid of which the measuring voltage (UM) is compensated at high temperatures, is performed with the aid of a high temperature compensation stage (DT) and of a converter circuit (UMS).

PRIOR ART

The invention is based on a device for determining the throughput of aflowing medium, for example the aspirated amount of air of an internalcombustion engine, in accordance with the species of the main claim.

Sensors, as well as associated evaluation circuits, by means of whichthe throughput of a flowing medium can be determined, are known fromDE-OS 43 24 040, for example. With these known mass flow sensors thesensor element is exposed to the flowing medium, for example the airflow. Here the sensor element comprises a heater, which is brought to anhigh temperature in relation to the medium to be detected by theapplication of a controlled current. A heater temperature sensor, aswell as a temperature sensor, which detects the temperature of theflowing medium, are associated with this heater. Twotemperature-dependent resistors are located in the spatial vicinitywhich, in respect to the flow direction of the medium to be detected,are placed laterally to the heated resistor, so that they are evenlyheated by the latter. But they are cooled at different rates by theflowing medium, since the resistor reached first by the flow is cooledmore than the other. The temperature difference resulting from thisprovides a measured voltage at a diagonal of the bridge of the proposedbridge circuit. The mass of the flowing medium is determined as afunction of this measured voltage.

A device for determining the throughput of a flowing medium isdescribed, wherein a heater resistor heats at least one upstream anddownstream located, temperature-dependent resistor, wherein theresistors, which are cooled at different rates by the flowing medium,are wired to form a bridge, whose diagonal voltage is a function of thetemperature difference of the resistors. In the end the mass of theflowing medium is determined from this processed voltage (UM). Thecontrol of the temperature of the resistor is performed with the aid ofits own heater control circuit which regulates the current through theheater resistor in such a way that the temperature of the heaterresistor is a predeterminable function of the temperature of the flowingmedium. A temperature compensation, with the aid of which the measuringvoltage (UM) is compensated at high temperatures, is performed with theaid of a high temperature compensation stage (DT) and of a convertercircuit (UMS).

ADVANTAGES OF THE INVENTION

The device in accordance with the invention for determining thethroughput of a flowing medium with the characteristics of the mainclaim has the advantage in comparison with the known device that afurther improvement of the temperature behavior is achieved. Thisapplies in particular to the high temperature behavior of the claimeddevice. This advantage is obtained in that the known circuit arrangementis complemented by an additional resistor bridge circuit, wherein thisadditional resistor bridge circuit permits high temperaturecompensation.

Further advantages of the invention are obtained by the steps recited inthe dependent claims.

DRAWINGS

An exemplary embodiment of the invention is represented in the drawingsand will be explained in more detail in what follows. The single drawingfigure here shows the entire circuitry of the device in accordance withthe invention with a heater control circuit and a so-called Delta Tbridge circuit, such as is known at least in part from DE-OS 43 42 040.In addition, the circuit in accordance with the drawing figure alsocontains the additional high temperature compensation stage.

DESCRIPTION OF THE EXEMPLARY EMBODIMENT

A device in accordance with the invention for determining the throughputof a flowing medium is represented in the drawing figure, wherein theentire arrangement is placed on a substrate and is exposed in a suitablemanner to the medium to be detected, for example the airflow in anaspirating tube of an internal combustion engine.

The heater control circuit HK comprises a bridge circuit with theresistors R1, R2, R3, RHF and RLF, wherein RHF is the heater temperaturesensor and RLF the air, or respectively medium temperature sensor, andthe value of these resistors is a function of the temperature. Theresistor bridge of the heat control circuit HK is located between avoltage UK and ground.

Heating takes place with the aid of a heat resistor RH, which isconnected to the battery voltage UB via the collector-emitter path of atransistor Ti and a resistor R10, wherein a Zener diode is additionallyswitched between the resistor R10 and ground for compensation of thevoltage. The other side of the heater resistor RH is connected withground via a resistor R5.

The second diagonal of the bridge circuit of the heater control circuitis connected with the two inputs of an operational amplifier OP1, whoseoutput leads to the base of the transistor T1. Another resistor R4 islocated between the inverting input of the operational amplifier OP1 andthe heater resistor RH, or respectively the emitter of the transistorT1.

The actual measuring circuit is a resistor bridge circuit with theresistors RAB1, RAB2, RAU1, RAU2 and RP, designated as a temperaturedifference bridge (Delta T bridge circuit) DT. These resistors aretemperature-dependent resistors which, the same as the heat resistor RHand the heater temperature sensor RHF, are at a high temperaturecompared with the temperature of the medium. Here, the resistors RAB1and RAB2 are arranged downstream of the heater resistor, but theresistors RAU 1 and RAU2 upstream thereof, in relation to the flowdirection of the medium to be detected.

The resistor bridge of the Delta T circuit DT is connected by onediagonal with the output of an operational amplifier OP2, whosenon-inverting input is supplied with a reference voltage UR via aresistor R20, another resistor R14 is located between the resistor R20and ground. The other side of the bridge diagonal mentioned is alsoconnected with ground.

The voltage occurring at the other bridge diagonal is coupled out withthe aid of a potentiometer P1, which lies parallel with the resistor RP.The wiper connector of the potentiometer P1 leads to the non-invertinginput of an amplifier OP3, whose inverting input is connected with theconnection point between the resistors RAB1 and RAU1. The amplifier OP3is an amplifier with adjustable amplification. Digital voltagecompensation is performed with the aid of the circuitry block VA, whichhas three connectors PR, DA, TA, through which the required triggeringsignals are supplied by an external evaluation device.

Incidentally, the amplifier OP3 is further connected via a voltagedivider R21, R22 with the output as well as the inverting input of theoperational amplifier OP2. The output of the amplifier OP3, where themeasuring voltage UM can be picked up, leads to a resistor combinationR7, R6, R13, which is a component of the converter circuit UMS, which isstill to be described.

Besides the already mentioned resistors, the converter circuit UMScomprises an operational amplifier OP4, whose non-inverting input isconnected with the connecting point between the resistors R6, R7, R13,and whose output is connected via a resistor R9 with the invertinginput. The output voltage UA is generated at the output of theoperational amplifier OP4, which is employed for determining the mass ofthe flowing medium, with the aid of the converter circuit UMS, whichleads to the high temperature compensation stage HTA via the resistorR8, the voltage UM, which is provided by the measuring bridge, issuperimposed on the compensation voltage generated in the compensationstage. The high temperature compensation stage HTA comprises theresistors R11, R12 as well as the temperature-dependent resistors R15,R16, which are wired as a bridge, and on the one hand are connected withthe output of the operational amplifier OP2 as well as the respectiveconnection of the DT bridge, while the other side of the bridge isconnected to ground.

Two potentiometers P2, P3 are located in the other bridge diagonal,wherein one potentiometer connection is fixed and the other adjustable.The fixed connector of the potentiometer P3 leads via the resistor R8 tothe converter circuit UMS. The variable wiper connector of thepotentiometer P2 leads via the resistors R6, R7 of the converter circuitUMS to the output of the operational amplifier OP3, to which themeasuring voltage UM is applied.

Of the total arrangement represented in the drawing figure, the hightemperature compensation stage HTA and the converter circuit are themost important components of the instant invention, while the heatercontrol circuit HK and the DT bridge are essentially already known fromDE-OS 43 24 040.

To describe the mode of operation of the entire device, first the modeof functioning of the heater control circuit HK and of the DT bridgewill be discussed, thereafter the mode of functioning of the HTAcompensation stage and the converter circuit UMS will be described.

The resistor RH, i.e. the heater resistor, the resistor RHF, which isintended to detect the temperature of the heater resistor, and theresistors RAU and RAB which, in the exemplary embodiment are designed asseparate resistors RAU1, RAU2, RAB1, RAB2, are arranged on thesubstrate, for example a diaphragm. In this case the heater resistor RHis completely surrounded by the resistor RHF, the resistors RAU and RAB,or respectively RAU1, RAU2 and RAB1, RAB2 are respectively disposedlaterally of the resistor RH and RHF. The flow of the medium, which isindicated in the drawing figure by an arrow as well as theidentification m, occurs in such a way, that the flow first flows overthe resistor RAU, or respectively RAU1, RAU2, and then over the resistorRH and thereafter over the resistor RAB, or respectively RAB1, RAB2. Bymeans of this the resistor RAU is cooled more than the resistor RAB. Thedifference in cooling is evaluated for determining the flowing medium.

The resistor RHF, which measures the temperature of the substrate, orrespectively the diaphragm, in the vicinity of the heater resistor RH isarranged in a bridge circuit, as represented in the drawing figure. Thebridge arm in which the resistor RHF is arranged is connected with theinverting input of the operational amplifier OP1. The output voltage ofthe operational amplifier OP1 acts on the base of the transistor T1 insuch a way, that a feedback to the heater resistor RH takes place insuch a way that heating of the resistor RHF leads to a reduction of thecurrent flow to the resistor RH.

The resistor RLF, whose resistance is a function of the air temperature,is arranged in the other bridge arm. Since this resistor arm acts on thenon-inverting input of the operational amplifier OP1, the flowconduction is generally increased by the heater resistor RH withincreasing temperature of the air, or respectively the flowing medium.The two bridge arms therefore act in such a way on the heater resistor,that a defined high temperature in comparison with the air temperatureis set, i.e. a defined temperature difference between the airtemperature and the heater temperature.

An additional temperature dependence of the high temperature can begenerated by a suitable selection of the resistors R1 and R3, which arealso arranged in the bridge arm which retroacts on the invertingoperational amplifier input. In this case the high temperature of theheater resistor RH in comparison with the air temperature is notconstant, but is a function of the air temperature. The resistors R1 andR3 should be designed a platinum resistors, for example, whoseresistance changes as a result of the air, or respectively medium,temperature. This additional dependence of the temperature of the heaterresistor on the ambient temperature can be used for compensating forsecondary effects, such as the temperature dependence of the heatconducting ability, the density or similar effects. In this way it ispossible by an appropriate selection of the resistor values of theresistors R1, R3, RHF to achieve a particularly linear outputcharacteristic curve of the sensor. The two resistors RAU (upward) andRAB (downward), or respectively the arrangement RAU1, RAU2 and RAB1,RAB2 represented in the drawing figure, are provided for evaluating themass flow. If no flow moves along the top of the substrate, theresistors RAB and RAU are evenly heated by the heater resistor RH. Witha flow on the top of the substrate, the upward located resistor, orrespectively the upward located resistors RAU1, RAU2, are cooled morestrongly than the downward located resistors RAB or respectively RAB1,RAB2. It is even possible that the temperature of the downward locatedresistor RAB is increased by a flow, since the flow transports heat fromthe heater resistor RH to the downward located resistor.

The resistors RAU1 and RAB1 as well as RAU2 and RAB2 are arranged in abridge circuit, which is compensated with the aid of the resistor RP aswell as the parallel connected potentiometer P1, which is triggered bythe operational amplifier OP3. Optimal compensation is possible, becausethe resistance behavior of the potentiometer P1 can be changed at anytime and from the outside by an external action on the operationalamplifier P3. The amplifier OP3 is influenced via a compensation stageVA, by means of which a digital amplifier compensation can be performed.To this end, signals are supplied to the compensation stage VA via theinputs PR (program) and DA (data), clock signals reach VA via TA.

The high temperature compensation stage HTA is introduced for thefurther improvement of the properties of the circuit arrangement so fardescribed. The resistor bridge circuit with the resistors R11, R12, R15,R16, of which the resistors R15, R16 have the same temperaturedependence, for example, is the main component for temperaturecompensation. At room temperature the resistor ratio of R11/R15 is equalto R12/R16 should apply. Therefore the diagonal voltage of the bridge atroom temperature equals zero millivolt.

Because of the temperature dependency of the resistors R15 and R16, adiagonal voltage UD is generated at other temperatures, which is notequal to zero and is proportional to the temperature.

The two potentiometers P2 and P3 of the high temperature compensationstage HTA are arranged in the transverse arm of the resistor bridge. P3here is fixedly set to the center position, i.e. the potentiometerresistance to the left and the right of the wiper is identical.

The wiper position at the potentiometer P2 is variable. It is possibleby means of this potentiometer P2 to set the proportion needed for thecompensation of the temperature error. This setting is provided startingwith the output voltage UM at the operational amplifier P3. With optimaldecoupling from the resistors R8 and R6 it is possible to create avoltage which is a function of the temperature, wherein the compensationat high temperatures does not affect the compensation at roomtemperature if, as already mentioned:

R11/R15=R12/R16 applies with the condition that room temperatureprevails. Incidentally, the uncoupling from the resistors can berealized in that high-impedance resistors R8, R6 are employed, or avoltage sequencer circuit is employed.

It is possible by means of the high temperature compensation stage tokeep the effect of the media temperature on the measured result low,particularly in connection with a low flow. It is not necessary herethat the temperature be constant during the high temperaturecompensation, it is merely required that a sufficient temperatureincrease in comparison with the room temperature is provided. Thecircuit arrangement represented in the drawing figure, in particular inthe area of the heater control circuit HK or the temperature differencebridge DT, has been provided by way of example, other, similarly actingcircuit arrangements are possible. As already mentioned, it is possible,for example, to combine the resistors RAB1 and RAB2, as well as theresistors RAU1 and RAU2 in respectively one resistor. It is furthermorepossible to connect compensation resistors in series in addition to theheater temperature sensor RHF, wherein the former can be respectivelylocated upstream and downstream of the bridge arm.

For example, the resistor RP can be omitted in the DT bridge circuit andonly one potentiometer P1 can be replaced. The portion of the entireinstallation identified by IC1 is a hybrid circuit, for example, whichincludes a plurality of operational amplifiers, current sources andtransistors, as well as a number of hybrid resistors with selectabletemperature coefficients.

The entire arrangement represented in the drawing figure can beintegrated in a chip with connections, at which the following values aresupplied, or respectively picked up:

UB: Battery voltage

UR: Reference voltage

PR: Program for amplification control

DA: Data input

TA: Clock frequency

UA: Output voltage

GND: Ground connection

I claim:
 1. A device for determining the throughput of a flowing medium,having a substrate, which can be exposed to the flowing medium, on whicha first resistor arrangement (R1, R2, R3, R5, RH, RHF), which is acomponent of a heat control circuit, is arranged, with at least oneresistor RH, which is heatable to a predeterminable temperature, and asecond resistor arrangement (RAB1, RAB2, RAU1, RAU2), which is wired asa bridge, whose one diagonal is located between a supply voltage andground and includes at least two temperature-dependent resistors (RAB1,2, RAU1, 2) which, in relation to the flow direction of the flowingmedium to be detected, are arranged above and below the heater resistor(RH), so that they are evenly heated by it, while they are being cooledat different rates by the flowing medium, and the measuring voltage (UM)which occurs as a result of the temperature difference is evaluated atthe other bridge diagonal for determining the throughput, characterizedin that a third resistor arrangement (HTA) is provided, which comprisesresistors (R11, R12,) temperature-dependent resistors (R15, R16) in abridge circuit and at least two potentiometers (P2, P3), which arelocated in the transverse arm of the bridge, and which detects adiagonal voltage, which is a function of the temperature and issuperimposed by the measuring voltage.
 2. The device in accordance withclaim 1, characterized in that one of the potentiometers of the thirdresistor arrangement is fixedly set and the other is variable.
 3. Thedevice in accordance with claim 2, characterized in that thepotentiometer which can be set to be variable is adjustable as afunction of the measuring voltage (UM), wherein the setting of theresistance ratio of the potentiometer takes place as a function of theoutput signal of the operational amplifier (OP3), which the measuringvoltage (UM).
 4. The device in accordance with claim 2, characterized inthat the setting of the potentiometer is provided in such a way that anoptimal compensation of the temperature error takes place at highertemperatures.
 5. The device in accordance with claim 1, characterized inthat the diagonal voltage (UD) of the third resistor arrangement and themeasuring voltage (UM) are superimposed in a converter circuit (UMS) forgenerating the temperature-compensated output signal (UA).
 6. The devicein accordance with claim 1, characterized in that the resistors of thethird resistor arrangement (HTA) have been selected such that a roomtemperature R11/R15=R12/R16 applies.
 7. The device in accordance withclaim 1, characterized in that the resistors (R6 and R8) are highimpedance ones or that the operational amplifier (OP4) is wired as avoltage sequencer.